Conversion of hydrogen iodide to iodine



United States Patent CONVERSION OF HYDROGEN IODIDE T IODKNE Charles R.Greene, Berkeley, and Shelton Steinle,

Richmond Annex, Califi, assignors to Shell Development Company, NewYork, N.Y., a corporation of Delaware No Drawing. Application October26, 1956 Serial No. 618,456

8 Claims. (Cl. 23-216) This invention relates to a process forrecovering elemental iodine from hydrogen iodide. More particularly,this invention relates to a process for oxidizing hydrogen iodide toiodine, employing molecular oxygen as the oxidizing agent, in thepresence of a catalyst.

We have discovered that the rate at which hydrogen iodide reacts withmolecular oxygen is markedly increased by conducting the reaction in thepresence of a solid material which has substantial intrinsic surfaceacidity. Thus, we have found that substantially complete conversion ofhydrogen iodide to iodine can be effected at very high rates attemperatures of about 300 C. or less by reacting hydrogen iodide withmolecular oxygen in the presence of a solid material having asubstantial intrinsic surface acidity.

Further, we have found that these acidic solid materials etfectivelycatalyze the reaction of hydrogen iodide with molecular oxygen whetherthe reaction is carried out in the vapor phase, or in the liquid phase.

Our discovery thus provides basis for a highly flexible, practicalprocess for converting hydrogen iodide to iodine. In its broad aspect,this new process comprises intimately contacting a molecularoxygen-containing gas with hydrogen iodide in the presence of a solidmaterial having a substantial surface acidity and thereafter recoveringthe product iodine from the reaction zone.

The use as catalyst of a solid material having substantial intrinsicsurface acidity constitutes the fundamental basis of the new process. Bythis is meant solid materials which in and of themselves exhibitprotonic surface activity. Not included are solid materials which are ormay be acidic on their surface because of the presence of a strongmineral acid adsorbed or absorbed on that surface.

It has been found that the intrinsic surface acidity of a solid materialcan be expressed in terms of an acidity function, usually designated HSee Hammett, Physical Organic Chemistry, McGraw-Hill, 1940, pages 266 etseq., and Hine, Physical Organic Chemistry, McGraw- Hill, 1956, at pages59-61. The intrinsic surface acidity of a solid material is usuallyexpressed by a numerical value given by the equation.

B) ar?) in which pK is the acid ionization constant for the conjugateacid BH+ of a neutral base (i. e., proton acceptor) B, C is theconcentration of the base, B, and C is the concentration of theconjugate acid, BH+, referred to dilute aqueous solutions. It will beseen that the intrinsic acidity of an acid is numerically equal to theacid ionization constant for the conjugate acid of the neutral base whenone half is in the form of the conjugate acid BH+ and one half is in theform of the base B. It has been found that the numerical value for thepK can be determined through the use of a neutral base of suitablebasicity which shows a visible change of color upon neutralization withthe acidic solid Intrinsic surface aeidity=H =pK +log ice material.Thus, the numerical value for the H, of a given material can bedetermined by applying to it a small amount of a solution in an organicsolvent of an indicator having a previously determined pK value andobserving the color of the solid containing the adsorbed indicatorcompound. This technique for determining the intrinsic surface acidityof a solid material is described generally in Hammett, supra, beginningat page 271, and both the theoretical basis for the use of thetechnique, and a detailed description of the technique is given incopending application Serial No. 387,512, filed October 21, 1953 whichmatured into U.S. Patent No. 2,868,688, issued January 13, 1959. Thepertinent disclosures of copending application Serial No. 387,512,relating to the meaning and determination of the intrinsic surfaceacidity of a solid material, are hereby specifically incorporated intoand made a part of this description of our invention. I

It has been found that for a solid material to be effective as acatalyst for the reaction of hydrogen iodide with molecular oxygen, theintrinsic acidity of that material rnust correspond to a H of less thanabout 2.0. It has also been found that the higher the intrinsic acidityof a material, i.e., the lower (more negative) its Ho, the bettercatalyst that material will be. It is preferred that the intrinsicacidity of the solid material used as catalyst correspond to a H of lessthan about 0.0.

Because of their availability and stability the argillaceous materialshaving substantial intrinsic surface acidity as defined above arepreferred as the catalyst. By argillaceous material is meant anymaterial, natural or synthetic, which has the properties commonlyassociated with clays, clayey materials and/or the ceramic materialsresulting from sintering, calcining or otherwise heat treating naturalclays or clayey materials, or synthetic mixtures of silica (usually inthe form of a gel) and alumina, magnesia, zirconia, or other materials.Thus, the term argillaceous material includes the naturally occurringclays, such as the montmorillonite clays, the kaolinite clays, theattapulgite clays, the hydrated micas, and the like. Also included arethe chemically and/or physically modified natural clays, such as theactivated natural clays, the acid-leached natural clays, and naturalclays which have been subjected to heat treatment, such as sinteredclays, calcined clays, and the like, and natural clays which have beensubjected to a combination of physical and chemical treatments. The termargillaceous material also includes the various synthetic materialswhich have the characteristics of natural clays or the ceramic materialsderived therefrom. These claytype materials consist largely of silicaand/or alumina and/ or magnesia, and are often modified by inclusion ofminor amounts of such materials as B 0 ZrO and the like. These syntheticmaterials ordinarily are available in the form of hard particles orgranules, prepared by heat treatment of intimate mixtures of the variouscomponents. The term argillaceous material also is intended to includesubstantially pure alumina and silica, whether in natural form, such asin bauxite, or chemically pure A1 0 or SiO provided the form of thesematerials is such that they have a substantial intrinsic surfaceacidity. The argillaceous material per se may be used as the catalyst,or there may be used a composite solid catalyst in which theargillaceous material is but one component, and may be either the majorcomponent, or but a minor component of that composition.

In general, argillaceous materials suitable as catalyst for the reactionof hydrogen iodide with molecular oxygen are those argillaceousmaterials which are known to be catalysts for the cracking ofhydrocarbons. Natural clays suitable for this purpose thus include thekaolinite clays,

such as kaolinite, nacrite, dickite and anauxite, the attapulgite clays,such as attapulgite and sepiolite, the montmorillonite clays, such asmontmorillonite, saponite, montronite andbeidelite, and other silicates,such as talc, mica, and pyrophillites which have been suitably treated(chemically, physically or both chemically and physically) to preparethem for use as hydrocarbon cracking catalysts. Synthetic clay-typecracking catalysts also are suitable, including the various combinationschosen from silica, alumina, zirconia, boria and/or magnesia. Thecombinations of silica with alumina and silica with magnesia are ofparticular interest. The activated forms of silica and alumina also maybe used as the catalyst.

The new process is effective for converting hydrogen iodide to iodineregardless of the source of the hydrogen iodide. That is to say, purehydrogen iodide may be used in the new process, or the hydrogen iodidemay be merely one component of a mixture of compounds. From thestandpoint of practical operating efficiency, it is desirable, ofcourse, that the hydrogen iodide concentration in the reaction zone beas high as economically feasible.

The conversion of hydrogen iodide to iodine may be carried out in thevapor phase, or it may be carried out in the liquid phase. Where theconversion is carried out inthe vapor phase, and the hydrogen iodide isbut one component of a mixture of gases, the part of the mixture otherthan hydrogen iodide may be composed of any material or materials whichare substantially inert in the reaction zone. Thus, inert diluents suchas nitrogen, helium or other of the inert gaseous elements, carbondioxide, or other inert gaseous inorganic compounds, or the like, may bepresent. Also, there may be present gaseous organic materials which arenot reactive with any one or all of hydrogen iodide, iodine, water, ormolecular oxygen in the presence of an acidic solid material at thetemperatures employed. It has been found that the presence ofsubstantial amounts of either or both of water and iodine in the mixtureto be treated will not adversely afiect the conversion of hydrogeniodide to iodine, despite the fact that it might be expected that, sinceboth compounds appear on the right-hand side of the reaction equation,the presence of either or both of water or iodine might inhibit or limitthe desired reaction.

The conversion of hydrogen iodide to iodine according to our discoveryalso may be carried out in liquid phase. Thus, liquid hydrogen iodidemay be oxidized to iodine by this new process. In most cases, however,it

will be more convenient to dissolve the hydrogen iodide in water andsubject this aqueous solution to contact with molecular oxygen accordingto the new process. The presence of aqueous water does not. appear toinhibit the desired reaction significantly. Also, the presence ofsubstantial amounts of iodine in the reaction mixture does not appear toinhibit or limit the desired reaction. However, it may be desirable tolimit the amount of iodine in the reaction mixture. It is preferred thatwhere iodine is present the molar ratio of iodine to hydrogen iodide notexceed one.

Molecular oxygen from any source may be used. Thus, pure molecularoxygen is suitable, as are mixtures of molecular oxygen with othergases, such as commercially pure (95%) oxygen, oxygen-enriched air, orair itself.

Where conversion of the hydrogen iodide is to be efiected in the vaporphase, the amount of molecular oxygen used preferably is at least theamount theoretically required to convert all of the hydrogen iodidepresent in the reaction zone to iodine. In some cases, it may be foundconvenient and desirable to use somewhat less than the theoreticalminimum amount of molecular oxygen. Generally, however, to insuremaximum conversion of hydrogen iodide to iodine, it is desirable thatthe amount of molecular oxygen fed be moderately in excess of thetheoretical minimum. In such cases, the

excess of oxygen should amount to at least 10% over that theoreticallyrequired, and it is preferred that at least a 50% excess of oxygen bepresent in the reaction zone. A large excess of oxygen is not necessary,and in most cases will be undesirable because it is uneconomical.Usually, little advantage will accrue from the use of more than about a500% excess of oxygen, and in most cases it is preferable that theamount of oxygen exceed the amount theoretically required to oxidize allof the hydrogen iodide present by from about 50% to about 350%. When airor other source of molecular oxygen containing an inert diluent gas isused, precaution should be taken to insure that there is a substantialproportion of each of hydrogen iodide and molecular oxygen in thereaction zone.

Where the conversion of hydrogen iodide to iodine is to be etfected inthe liquid phase, somewhat greater excesses of molecular oxygen usuallyare required than when the conversion is to be carried out in the vaporphase. Thus, it normally will be found necessary, when conducting thereaction in the liquid phase, to maintain at least about a 25% excess ofmolecular oxygen in the reaction zone, and in some cases as much as aZOO-fold excess of oxygen will be found desirable. Preferably, theexcess of oxygen is at least 50%; an excess of more than about lOO-foldis not often required, for such large excesses provide little advantageover somewhat lesser excesses and are usually uneconomic and presentoperating difficulties.

When operating in the liquid phase, it is essential to the effectiveoxidation of hydrogen iodide that there be a substantial partialpressure of molecular oxygen in the reaction zone. Thus, the oxygenpartial pressure should be at least 10 p.s.i., and optimum oxidationrates are usually obtained only when the oxygen partial pressure is 20p.s.i. or more. While much higher oxygen partial pressures may beusedfor example, up to 200 psi. or even more-in general, little addedadvantage results from the use of oxygen partial pressures in excess ofabout p.s.i.

A primary factor in effecting the reaction between hydrogen iodide andmolecular oxygen in the presence of a liquid phase at practical rates isthe maintenance of intimate contact between the gas and liquid phases;practical reaction rates can be obtained only when a very high degree ofcontact between the gas and liquid phases is maintained. Means forobtaining and maintaining intimate contact between gases and liquids arewell known in the art. Any of the known methods may be used in theprocess of this invention. For example, the reaction mixture may bestirred or otherwise thoroughly agitated, or the liquid materials may bepassed in countercurrent flow to the gaseous materials in a tower packedwith an inert packing or the catalyst, or in a tower equipped withdevices for insuring intimate gas-liquid contact, including towersequipped with grid trays, bubble plates, rotary disc contactors or thelike. The particular method chosen should be, of course, capable ofeifecting the necessary degree of contact in the presence of the solidcatalyst. The catalyst may be in the form of a fine powder, or ingranular form, as may be most convenient and eflfective.

The conversion of hydrogen iodide to iodine is effected at anytemperature above about 50 0; however, the reaction rate increasessignificantly with temperature. When operating in the vapor phase, theminimum temperature is determined by one of two factors: first, if it isdesired that the product iodine be obtained in liquid phase, it will, ofcourse, be necessary to conduct the reaction at a temperature above themelting point of iodine (ll3.5 0.); second, if there is present in thereaction zone any material which condenses at a temperature above themelting point of iodine, or if it is desired to recover the productiodine as a solid and there is present in the reaction zone any materialwhich condenses at any temperature above about 50 C., then the dewpointof the material which will so condense fixes the minimum reactiontemperature. From the standpoint of the reaction of hydrogen iodide withmolecular oxygen to form iodine, per se, there is no maximum limit onthe temperature at which the reaction may be conducted. However,temperatures above about 400 C. will not be required, for at thistemperature level, and in most cases, at temperatures substantiallybelow this level, the reaction of hydrogen iodide with molecular oxygenproceeds at very high rates. Practically satisfactory reaction rates areobtained at temperatures substantially below 400 C., for example, attemperatures of from about 100 C. to about 300 C. Because of thesubstantial advantages obtained, from the standpoint of corrosion andthe useful materials of construction available, by conducting thereaction at as low a temperature as possible consistent with a feasiblereaction rate, reaction zone temperatures of from about 100 C. to about250 C. are most suitable. When operating in the liquid phase, themaximum temperature is, of course, that of the boiling reaction mixtureat the pressure used. It is normally desirable to conduct the reactionat temperatures somewhat lower than that at which the reaction mixtureboils, since boiling mixtures do not absorb gases (in this case,molecular oxygen) readily. Preferably, when the reaction is carried outin the liquid phase, the temperature is at least about 80 C.

The conversion of hydrogen iodide to iodine in the vapor phase may becarried out at any pressure. Operation at substantially atmosphericpressure is quite prac tical, and in a great many cases will be found tobe the most convenient operating pressure. Few, if any, substantialadvantages are obtained by operating at reduced pressure, but in manycases, it will be found both convenient and desirable to conduct thereaction at moderately elevated pressures. For example, pressures of upto about 500 p.s.i.g. may be used to reduce the volume of gases to behandled. Where the conversion is carried out in the liquid phase, theminimum pressure which can be used normally will be determined by theoxygen partial pressure desired. If pure oxygen is used, the systempressure need not be substantially greater than the oxygen partialpressure used. If air, or other mixture of oxygen with an inert gas isused, then the system pressure will be correspondingly greater tofurnish the requisite oxygen partial pressure. In some cases, the use ofelevated pressures may be desirable to reduce the volume of gaseshandled and/ or to increase the boiling temperature of the hydrogeniodide solution in the reaction zone. Pressures in excess of about 500p.s.i.g. will seldom be found advantageous or desirable, as compared tosomewhat lower pressures.

At the temperatures set out above, practically feasible hydrogen iodideconversion levels are obtained in a few seconds reaction time. Forexample, when operating in the vapor phase, at temperatures of fromabout 150 C. to about 200 C., using typical acidic argillaceousmaterials as catalyst, substantially quantitative conversion of hydrogeniodide to iodine is effected at residence times of the magnitude ofabout 1 to about seconds. At higher temperatures, the required residencetime is correspondingly lower. When operating in the liquid phase, withadequate means for insuring intimate contact between the gas and liquidphases in the reaction zone, substantially complete reaction is obtainedin from about 30 to about 60 minutes.

Recovery of the product iodine from the eifluent from the reaction zonemay be effected by known methods, the method used depending upon whetherthe conversion of hydrogen iodide to iodine was efiected in the liquidphase or in the vapor phase, upon the extent to which the hydrogeniodide was converted to iodine, and upon the nature of the components ofthe efiiuent other than iodine, water and hydrogen iodide, if any bepresent. If the conversion of hydrogen iodide to iodine is substantially100%,

and the conversion of hydrogen iodide was effected in aqueous liquidphase, the product iodine is immiscible with the aqueous phase and thetwo phases may be separated by decantation where the iodine is liquid,or by filtration, centrifuging or the like, where the iodine is solid.If the conversion of hydrogen iodide to iodine is substantially and theconversion of the hydrogen iodide to iodine was efiected in vapor phase,the iodine may be recovered most simply by cooling the efliuent vaporsto form liquid water and liquid or solid iodine, from which the iodineis recovered by phase separation as where the hydrogen iodide conversionwas carried out in aqueous liquid phase. Where the conversion of thehydrogen iodide is incomplete, the effluent will contain both iodine andhydrogen iodide. Where the conversion of hydrogen iodide was carried outin aqueous liquid phase, or in the aqueous liquid phase resulting fromcondensation of the efiiuent vapors where the conversion of hydrogeniodide was carried out in the vapor phase, the hydrogen iodide and, to acertain extent, the iodine will be dissolved in the aqueous phase. Inmany cases it will be found possible to so control the degree ofhydrogen iodide conversion and the amount of water present in theeffluent so that the amount of iodine formed exceeds substantially theamount of iodine which will dissolve in the hydrogen iodide solutionobtained from the eflluent. This permits direct removal of a substantialpart of the product iodine by simple phase separation. The iodinedissolved in the hydrogen iodide solution may be recovered by treatingthe solution with a strong oxidizing agent, such as chlorine, to convertthe remaining hydrogen iodide to iodine, and the iodine is separatedfrom the water by simple phase separation. Alternatively, the iodinedissolved in the hydrogen iodide solution may be recovered by passing aninert gas through the solution and recovering iodine from the efiiuentgases. This method for selectively removing iodine from mixtures ofiodine, hydrogen iodide and water is disclosed and claimed in copendingapplication Serial No. 594,893, filed June 29, 1956. Where the oxidationof hydrogen iodide is conducted in the liquid phase, this method may beused to advantage to recover iodine directly from the reaction mixture.Thus, it will be found that if a part of the gaseous portion of thereaction mixture be removed from the reaction zone, the gaseous materialcontains iodine and water, but no hydrogen iodide. Recovery of theiodine content of such mixtures is easily effected by the methodsalready set out herein.

This constitutes a general description of the process of the invention;the following examples illustrate specific applications of this process.,It is to be understood that these examples are for the purpose ofillustration only and that the invention is not to be regarded aslimited in any way to the specific conditions cited therein.

Example I A vaporous mixture of 27.5% by weight hydrogen iodide and72.5% by weight water was mixed with sufficient air to provide twice asmuch molecular oxygen as would theoretically be required to react withall of the hydrogen iodide and the entire mixture was continuouslypassed through a tubular reactor packed with a modified clay-bondedcalcined diatomaceous earth. This catalyst was prepared by digesting acommercially available claybonded calcined diatomaceous earth materialdesignated by the manufacturer, Johns-Manville Company, as Celite VIIIfor 16 hours under reflux with 25% by weight sulfuric acid. The amountof acid used was 1.5 times the bulk volume of the carrier. The leachedcatalyst was then drained, washed thoroughly with water, and dried in anoven at C. The material has an intrinsic surface acidity correspondingto an H, of from 3 to 5.5. The catalyst was in the form of pellets inchin diameter and A inch long. The temperature of the catalyst bed was somaintained that the temperature of the efliuent gases was C. Theapparent residence time was one. second. There was a conversion of 95%of the hydrogen.

iodide to iodine. When the reaction temperature was raised to 145 C.,97% of the hydrogen iodide was converted to iodine.

Example 11 A vaporous mixture of 23% by weight hydrogen iodide, 12% byweight iodine and 65% by weight water was mixed with sufficient air toprovide ten percent more molecular oxygen than would theoretically berequired to react with all of the hydrogen iodide. The entire mixturewas continuously passed through a tubular reactor packed with a modifiedclay-bonded calcined diatomaceous earth. This catalyst was prepared bytreating a commercially available clay-bonded calcined diatomaceousearth material designated by the manufacturer, Johns-Manville Company,as Celite VIII, as follows:

The diatornaceous earth material, 100 parts, in the form of pelletswhich had a generally cylindrical shape and measured approximately byinch, was soaked for approximately one hour, and at room temperature, inan excess of an aqueous solution of phosphoric acid containing 85% byweight H PO The excess acid was then removed by allowing the carriermaterial to drain for one hour. The impregnated carrier material wasthen placed in an oven and heated at 300 C. for 3 hours, the pressurebeing atmospheric, and the atmosphere surrounding the carrier materialcontaining a partial pressure of water equal to approximately 200 mm.Hg. The material was then cooled and leached. by digesting the materialfor one hour with acidified water maintained at 100 C. The water had aninitial pH of 0.35, the acidity being furnished by the addition ofsulfuric acid. The volume ratio of water to carrier material wasapproximately 1.5. The carrier material was then drained and theleaching repeated in an identical manner, using a fresh portion ofacidified water.

The acidified water was then drained from the material and the leachingrepeated twice more, following the same procedure, but substituting purewater for the acidified water.

The carrier material was drained, and dried in an oven at about 125 C.The product had an intrinsic surface acidity corresponding to an H of+1.5 to -3.5. The temperature of the catalyst bed was so maintained thatthe temperature of the effluent gases was 150 C. The apparent residencetime was 0.9 second. There was a conversion of 38.6% of the hydrogeniodide to iodine.

Example 111 A vaporous mixture of 27.5% hydrogen iodide and 72.5% waterwas passed with an amount of air providing a 200% excess of molecularoxygen over a bed of a commercially available bauxite catalystmaintained at a temperature of 127 C. Hydrogen iodide in an amount of55% was converted to iodine at a residence time of one second.Substantially quantitative conversion of the hydrogen iodide wasobtained in one second at a temperature of 157 C. The catalyst used wasa form of activated bauxite marketed under the trade name Porocel- S byAttapulgus Clay Company. It had an intrinsic acidity corresponding to anH of from about +1.5 to about 3.0. It had an approximate analysis:alumina, 91.3% by weight; silica, 7.2% by weight; titania, 1.5% byweight; 6.0% volatile matter. It had a surface area of 200220 squaremeters per gram. The catalyst particles would pass through an 8-meshscreen, but were held up on a 14-rnesh screen.

Example 1V Hydrogen iodide was oxidized to iodine in the liquid phase bythe following procedure. (In this description, parts by weight bear thesame relationship to parts by volume as does the kilogram to the liter.)

A solution, parts by volume, of 20% by weight hydrogen iodide in waterwas placed in an autoclave Time After Beginning of Run at Which Samplewas Taken (minutes) Percent HI Converted The catalyst was amicrospheroidal silica-alumina hydrocarbon cracking catalyst marketed byAmerican Cyanamid Company under the designation Aerocat Synthetic FluidCracking Catalyst, Grade MS-A-Z. This catalyst has the composition (drybasis): 13.3% by weight alumina; 86.65% by weight silica, 0.04% byweight iron; and 0.01% by weight sodium (as Na O). It has a surface of650 square meters per gram. It has an average particle size of 59microns, no particles greater than -rnesh, and 98% of particles whichpass through a l00-mesh screen. Before the catalyst was used, it wascalcined at 450 C. for four hours, then cooled before use. The intrinsicsurface acidity of the catalyst corresponded to an H of less than 8.2.

We claim as our invention:

1. A process for oxidizing hydrogen iodide to iodine which comprisesreacting hydrogen iodide and molecular oxygen at a temperature of atleast about C. in the presence of an argillaceous material having anintrinsic surface acidity (H of less than about 2.0, said argillaceousmaterial being the sole catalyst.

2. A process for oxidizing hydrogen iodide to iodine which comprisesreacting hydrogen iodide with molecular oxygen at a temperature of fromabout 50 C. to about 400 C. in the presence of an argillaceous materialhaving an intrinsic surface acidity (H of less than about 2.0 as solecatalyst.

3. A process for oxidizing hydrogen iodide to iodine which comprisespassing a gaseous mixture comprising hydrogen iodide and molecularoxygen in intimate contact with an argillaceous material having anintrinsic surface acidity (H of less than about 2.0 as sole catalystmaintained, at a temperature of from about 50 C. to about 400 C., andrecovering iodine from the effluent gases.

4. A process for oxidizing hydrogen iodide to iodine which comprisespassing a gaseous mixture comprising hydrogen iodide and molecularoxygen in intimate contact with a catalyst consisting of an argillaceousmaterial having an intrinsic surface acidity (H of less than 0.0maintained at a temperature of from about 50 C. to about 400 C., andrecovering iodine from the eliiuent gases.

5. A process for oxidizing hydrogen iodide to iodine which comprisesintimately contacting at a temperature of at least about 50 C. anaqueous solution of hydrogen iodide with a molecular oxygencontaininggas in the presence of an argillaceous material having an intrinsicsurface acidity (H of less than about 2.0 as sole catalyst, the partialpressure of molecular oxygen in the reaction zone being at least tenpounds per square inch.

6. A process for oxidizing hydrogen iodide to iodine which comprisesreacting hydrogen iodide and molecular oxygen at a temperature of atleast about 50 C. in the presence of a modified clay-bonded diatomaceous9 earth material having an intrinsic surface acidity (H of less thanabout 2.0 as sole catalyst.

7. A process for oxidizing hydrogen iodide to iodine which comprisesreacting hydrogen iodide and molecular oxygen at a temperature of atleast about 50 C. in the presence of an activated bauxite having anintrinsic surface acidity (H of less than about 2.0 as sole catalyst.

8. A process for oxidizing hydrogen iodide to iodine which comprisesreacting hydrogen iodide and molecular oxygen at a temperature of atleast about 50 C. in the presence of a silica-alumina hydrocarboncracking catalyst having an intrinsic surface acidity (H of less thanabout 2.0 as sole catalyst.

16 References Cited in the file of this patent UNITED STATES PATENTSOTHER REFERENCES Thorpe: Dictionary of Applied Chemistry, vol. II,

0 pp. 18-19 (1912), publ. by Longmans, Green and Co.,

New York, NY.

Latimer: Reference Book of Inorganic Chemistry, 3rd ed., p. 165 (1951),The MacMillan Co., New York, NY.

1. A PROCESS FOR OXIDIZING HYDROGEN IODIDE TO IODINE WHICH COMPRISESREACTING HYDROGEN IODIDI AND MOLECULAR OXYGEN AT A TEMPERATURE OF ATLEAST ABOUT 50*C. IN THE PRESENCE OF AN ARGILLACEOUS MATERIAL HAVING ANINTRINSIC SURFACE ACIDITY (HO) OF LESS THAN ABOUT 2.0, SAID ARGILLACOUSMATERIAL BEING THE SOLE CATALYST.